Book marker



FIG. 1 is a front view of a book marker disposed in a first position, according to the new design;

FIG. 2 is the front view of the book marker of FIG. 1 disposed in a second position;

FIG. 3 is a rear view of the book marker of FIG. 1 disposed in the first position;

FIG. 4 is the rear view of the book marker of FIG. 3 disposed in the second position; and,

FIG. 5 is a rear perspective view of the book marker of FIG. 1.

The broken lines shown in FIGS. 2 and 4 represent fold lines that form part of the claimed design. 

The ornamental design for a book marker, as shown and described. 